All through the night of April 15, 1848, slaves slipped out of their masters’ houses and crept through the streets of Washington, D.C. Their destination was the Pearl, a schooner that promised freedom to as many people who could fit on board. As more and more people filled the boat—77 in all—hope surged through the assembled slaves and the boat’s white crew. Freedom was just 225 miles away…that is, if they made it that far.

Now known simply as the Pearl Incident, the plot was one of the most daring of its era, and one of the most infamous. It was the largest attempted slave escape in American history—one that was doomed from the start.

The slaves’ decision to board the Pearl was not spontaneous; it was the product of months of planning. It was the brainchild, in part, of two free black men who had seen slavery firsthand. And though the stakes were high, the potential payoff was more than worth it.

Paul Edmonson knew the risks and rewards well. He was free, but his wife, Amelia, wasn’t—and Maryland law meant that all 14 of the children he had with his enslaved wife belonged to her mistress, Rebecca Culver. Though four of his children had purchased their freedom, the rest were still enslaved and their labor leased out to rich D.C. families.

Paul Jennings, who had been the slave of President James Madison’s family until his wife, Dolley, freed him in her will, was also active in the city’s anti-slavery movement, and the two men reached out to William Chaplin, one of Washington’s most prominent abolitionists. Chaplin and others, including Gerrit Smith, a philanthropist known for using his riches to fund anti-slavery efforts and assist African-Americans, agreed to fund the plan.

Captain Daniel Drayton, hated slavery, and during years of sailing up and down the Atlantic coast, the pleas he heard from enslaved people touched his heart. “Why had not these black people, so anxious to escape from their masters, as good a light to their liberty as I had to mine?” he wrote in a memoir of the incident. Drayton hired the Pearl  as the escape vessel, enlisting the ship’s white captain, Edward Sayres, and a single boatman to assist with the escape.

News spread that the ship would depart on April 15, and slaves hatched plans to head to the wharf on the Potomac that night. Among them were six Edmonson siblings, including 13-year-old Mary and 15-year-old Emily.

But though hopes were high, the tide—literally—was against the escape attempt. As the boat slipped into Chesapeake Bay on its way to New Jersey, a free state, it faced a strong headwind and the tide brought the ship to a halt within hours. The Pearl was forced to drop anchor near Point Lookout, Maryland.

Hours later, a posse hired by the slaves’ furious owners rendezvoused with the boat. They dragged the ship, slaves and crew back to Washington. “All on board were…made prisoner without bloodshed, although it was evident that the slaves would have resisted if there were any chance of escape,” wrote a local newspaper.

But the real danger awaited them in Washington, D.C., where an angry mob had gathered at the dock.The slaves, many of them in manacles and chains, were paraded through the streets. The mob taunted and threatened them and yelled obscenities at Drayton and his collaborators.

Then they turned on the nearby office of an abolitionist paper and threatened an openly abolitionist Congressman they accused of supporting the escape. The riot that ensued lasted for three days.

The aftermath was brutal for the slaves who dared to escape. All of them were sold to plantations further south as punishment—a common practice that ensured hard labor and separation from their families. Drayton and Sayres were tried, convicted of 77 counts of illegally transporting a slave and aiding a slave, and thrown into jail when they could not pay their fines. They only left jail four years later, when President Millard Fillmore, who was sometimes accused of being an abolitionist by his Southern enemies, pardoned them.

The fates of all 77 slaves are not known, but at least two of them eventually gained freedom. Paul Edmonson used the publicity of the Pearl disaster to raise money for his daughters’ release, and in November 1848 they were emancipated with funds raised by white abolitionists. Both spoke out against slavery and were educated, but Mary died tragically when she was just 20.

The escape was a catastrophe for the slaves who dared make a run for it. But ironically, their disastrous escape attempt helped end the slave trade in Washington, D.C. The Pearl incident and Washington Riot became so well known that pressure to stop the slave trade in the nation’s capital mounted. In 1850, aided by the publicity of the Pearl incident, Congress stopped allowing the import and sale of slaves into the District of Columbia. However, existing slaves in the District were still sold in the city’s thriving slave market.

The Pearl incident helped stop slavery in another way, too: Harriet Beecher Stowe, the famous abolitionist author, cited the failed escape as an inspiration for her book Uncle Tom’s Cabin. And that book helped shock America into abolishing slavery for good.